On Mon, 29 Aug 2016 14:31:20 +0800 Aaron Lu [off-list ref] wrote:

The global zero page is used to satisfy an anonymous read fault. If
THP(Transparent HugePage) is enabled then the global huge zero page is used.
The global huge zero page uses an atomic counter for reference counting
and is allocated/freed dynamically according to its counter value.

CPU time spent on that counter will greatly increase if there are
a lot of processes doing anonymous read faults. This patch proposes a
way to reduce the access to the global counter so that the CPU load
can be reduced accordingly.

To do this, a new flag of the mm_struct is introduced: MMF_USED_HUGE_ZERO_PAGE.
With this flag, the process only need to touch the global counter in
two cases:
1 The first time it uses the global huge zero page;
2 The time when mm_user of its mm_struct reaches zero.

Note that right now, the huge zero page is eligible to be freed as soon
as its last use goes away.  With this patch, the page will not be
eligible to be freed until the exit of the last process from which it
was ever used.

And with the use of mm_user, the kthread is not eligible to use huge
zero page either. Since no kthread is using huge zero page today, there
is no difference after applying this patch. But if that is not desired,
I can change it to when mm_count reaches zero.

I suppose we could simply never free the zero huge page - if some
process has used it in the past, others will probably use it in the
future.  One wonders how useful this optimization is...

But the patch is simple enough.

Case used for test on Haswell EP:
usemem -n 72 --readonly -j 0x200000 100G
Which spawns 72 processes and each will mmap 100G anonymous space and
then do read only access to that space sequentially with a step of 2MB.

perf report for base commit:
    54.03%  usemem   [kernel.kallsyms]   [k] get_huge_zero_page
perf report for this commit:
     0.11%  usemem   [kernel.kallsyms]   [k] mm_get_huge_zero_page

Does this mean that overall usemem runtime halved?

Do we have any numbers for something which is more real-wordly?


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